1. Field of the Invention
The present invention is generally in the field of night vision devices of the light amplification type. More particularly, the present invention relates to a night vision device having an improved image intensifier tube (I2T). A method of making such an improved image intensifier tube is disclosed also.
2. Related Technology
Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision can not utilize this infrared light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night-time scene.
Conventional night vision devices of the image intensification type (i.e., light amplification type) have been known for a considerable time. Generally, these night vision devices include an objective lens which focuses invisible infrared light from a night-time scene onto the transparent light-receiving face of an image intensifier tube. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an image intensifier tube with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons in a pattern replicative of the scene being viewed. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons results, which pattern replicates the input pattern but is at a considerably higher electron density. This pattern of electrons is moved from the microchannel plate to an output electrode which includes a phosphorescent screen. The phosphorescent screen produces a visible image available through a transparent image-output window of the tube.
A persistent problem with image intensifier tubes has been the presence of microscopic contamination within the tube which interferes with the proper operation of the tube. Despite the best possible cleanliness and efforts to control contamination, the manufacturing process itself for an image intensifier tube probably produces some of this microscopic particulate contamination, so that it cannot be entirely eliminated. The particulate contamination within the tube causes dark spots of various sizes on the image produced by the tube. This problem of dark spots in the image provided by image intensifier tubes has been so pervasive and long-standing that the industry and users of such tubes have come to accept this problem, and to deal with it not by eliminating the problem but simply by grading its magnitude. That is, so long as the number and size of dark spots in an image intensifier tube are below a certain standard, the tube is judged to be acceptable. That is not to say that the images provided by presently accepted image intensifier tubes are free of dark spots, but simply that they provide images that is good enough.